The present invention relates to a metallic article having a protective coating and a method of applying a protective coating to a metallic article. The present invention relates in particular to a nickel, a cobalt or an iron base superalloy article having a protective coating and a method of applying a protective coating to a nickel, a cobalt or an iron base superalloy article.
Conventional environmental protective coatings for nickel base superalloys, cobalt base superalloys and iron base superalloys include aluminide coatings, platinum modified aluminide coatings or chromium modified aluminide coatings for high temperature oxidation and Type 1 sulphidation resistance.
Conventional environmental protective coatings for nickel base superalloys, cobalt base superalloys and iron base superalloys include silicide modified aluminide coatings or chromised coatings for lower temperature Type 2 and Type 3 sulphidation resistance.
Aluminide coatings are generally applied by the well-known pack aluminising, out of pack vapour aluminising or slurry aluminising processes. Platinum coatings are generally applied by electroplating, sputtering or physical vapour deposition processes. Chromium coatings are generally applied by pack chromising or out of pack vapour chromising. Silicide coatings are generally applied by slurry aluminising.
It has been found that the roots, shanks and internal cooling passages of the turbine blades are suffering sulphidation, particularly low chromium nickel base superalloy turbine blades. The roots, shanks and internal cooling passages of the turbine blades may suffer from Type 2 and Type 3 sulphidation, this is a particular problem at low temperatures, below about 850xc2x0 C. The sulphidation may lead to stress cracking of the aerofoils and/or roots of the turbine blades.
In the case of turbine blades, or turbine vanes, for gas turbine engines it is known to provide more than one environmental coating if more than one type of oxidation or sulphidation is experienced. For example platinum aluminide coatings may be provided on the aerofoils of the turbine blades and chromised coatings may be provided on the shanks, roots and internal cooling passages of the turbine blades to provide environmental protection.
However, it has been found that for some metallic articles, that once the chromised coating has been penetrated by the sulphidation, the sulphidation of the underlying metallic article occurs at a greater rate than a metallic article without a chromised coating.
Accordingly the present invention seeks to provide a novel protective coating for a metallic article and a novel method of applying a protective coating to a metallic article which reduces, preferably overcomes, the above mentioned problem.
Accordingly the present invention provides a metallic article having a protective coating on the metallic article, the protective coating comprising a chromised coating on the metallic article and a glass coating on the chromised coating.
Preferably the glass coating comprises a silicate glass.
Preferably the glass coating is a silicate glass having a chromium oxide filler.
Preferably the metallic article comprises a nickel base superalloy, a cobalt base superalloy or an iron base superalloy.
Preferably the glass coating comprises a boron titanate silicate glass having a chromium oxide filler.
Preferably the metallic article comprises a turbine blade or a turbine vane.
Preferably the thickness of the chromised coating is 10 xcexcm to 30 xcexcm.
Preferably the chromised coating has an outer region, the outer region of the chromised coating comprises 20-30 wt % chromium.
Preferably the thickness of the glass coating is 5 xcexcm to 50 xcexcm.
The present invention also provides a method of applying a protective coating to a metallic article comprising chromising the metallic article and depositing a glass coating on the chromised metallic article.
Preferably the method comprises depositing a silicate glass on the chromised metallic article.
Preferably the method comprises depositing a silicate glass having a chromium oxide filler on the chromised metallic article.
Preferably the metallic article comprises a nickel base superalloy, a cobalt base superalloy or an iron base superalloy.
Preferably the method comprises depositing a boron titanate silicate glass having a chromium oxide filler on the chromised metallic article.
Preferably the metallic article comprises a turbine blade or a turbine vane.
Preferably the method comprises depositing the boron titanate glass and chromium oxide filler by spraying with a binder.
Preferably the method comprises drying the glass coating, heating the glass coating at 100xc2x0 C. for 1 hour and heating the glass coating at 1030xc2x0 C. for 10 to 20 minutes to fuse the glass coating. Preferably the thickness of the glass coating is 10 xcexcm to 50 xcexcm.
Alternatively the method comprises depositing the silicate glass by sol gel processing. Preferably the thickness of the glass coating is 5 xcexcm to 10 xcexcm.
Preferably the method comprises chromising the metallic article by pack chromising, out of pack vapour chromising, chemical vapour deposition, slurry chromising or physical vapour deposition.
Preferably the method comprises out of pack vapour chromising at a temperature of 1050xc2x0 C. to 1100xc2x0 C. for 1 to 6 hours.
Preferably the thickness of the chromised coating is 10 xcexcm to 30 xcexcm.
Preferably the chromised coating has an outer region, the outer region of the chromised coating comprises 20-30 wt % chromium.